This invention relates in general to polarization independent optical isolators and in particular to optical isolators which preserve the polarization state and the degree of polarization of light.
The use of fiberoptics in communications has developed rapidly in recent years with such progress, new problems have arisen. Thus, when a light source transmits light through an optical fiber to another optical device, the light transmitted through the optical fiber will be reflected by an end face of a fiber or other parts of the optical device at the other end so that such reflected light returns to the light source. Multiple reflections at the end faces of optical fibers or other optical devices can cause echoes. These effects adversely affect the performance of the source and compromise the information communicated in the fiber.
Various optical isolators and non-reciprocal devices have been developed to overcome the problem of light reflections and echoes described above. One such device is described in U.S. Patent application "Improved Optical Nonreciprocal Device" by Kok Wai Chang, Ser. No. 07/222,597, filed Jul. 21, 1988. Such application also describes other, types of prior art optical isolators or optical non-reciprocal devices. The isolator described in the above-referenced application employs walk off crystals. In other isolator designs, birefringent wedges are used, such as in U.S. Pat. No. 4,548,478 to Shirasaki. While the optical isolators described in the above-referenced patent application and patent are effective in overcoming the problem of light reflections Or echoes, such devices may give rise to other problems described below.
First it will be useful to define two terms used to describe the polarization of light. The polarization state of a light beam is defined by the magnitude and relative phase of any orthogonal pair of transverse components.
Where a light beam includes a component with a well defined polarization state and another component which is unpolarized light, the beam is said to have a degree of polarization given by the ratio of the power of the polarized light beam to the total power, which is the sum of the power of the polarized and unpolarized components of the beam.
Various lasers have been used for many purposes including communications and optical instruments. It is known that light from a laser will retain its polarization purity or coherence for a set distance characteristic of the laser where the distance is known as the coherent length of the laser. Thus, by definition, the output polarization state of the laser will remain the same within its coherent length.
The isolators described in the above identified application and patent are polarization independent isolators which are isolators which provide the same power light output irrespective of the polarization state or degree of polarization of the input light signal. This type of isolators has the advantage of eliminating the polarization dependent from mission of the light traveling in the forward direction.
When an optical isolator of any one of the types described in the above-referenced application or patent is used to reduce reflections when a beam is passed through an optical system, such isolator will introduce certain phase variations between the two polarization paths which may be temperature and wavelength dependent.
An optical isolator of any one of the types described in the above-referenced application or patent typically employs walk off crystals or birefringent wedges. Such crystals or wedges have different indices of refraction along at least two orthogonal axes. In a special case, when an input light beam with a linear polarization substantially parallel to one of the axes of the first walk off crystal or birefringent wedge is passed through any one of such types of isolators, most of the light will pass through each crystal or wedge of such isolator as one beam so that the polarization state and the degree of polarization of the light in the forward direction will be essentially preserved. Thus if a laser having a linear polarization state is passed through an isolator of such types in the above manner, the polarization state and degree of polarization will be preserved.
However, in situations other than the special case described above, an input beam of arbitrary polarization can be projected onto the orthogonal axes of the first walk off crystal or birefringent wedge of any one of- the isolators described in the above-referenced application or patent. And if such beam is passed through the crystal or wedge, such input beam will be separated by such crystal or wedge into an extraordinary ray and an ordinary ray corresponding to the two orthogonal axes having different indices of refraction. Since the two rays experience different indices of refraction through the crystals or wedges in the isolators, the two rays will separate. The isolators described in the above-referenced application or patent achieve optical isolation of reflections by taking advantage of such separation. Thus, an optical isolator would cause the extraordinary and ordinary rays to superpose each other in a forward direction when being transmitted to the desired location but would prevent reflections of the extraordinary and ordinary rays from superposing each other travelling in the reverse direction to overcome the problem of reflections and echoes.
From the above, except for the special case noted above, the input beam will be split into the ordinary and extraordinary rays travelling in the forward direction of the isolator. These two rays will experience different indices of refraction, so that the optical isolator introduces an optical path length difference between the two rays. Therefore, when the two rays superpose at the desired location in the forward direction, there will be an optical path length difference between the two rays. Hence, except for the special case, the above described isolators will introduce optical path length difference between the rays upon superposition.
When an optical isolator is used to reduce reflections in the transmission of laser light, and when the optical path length difference introduced by the isolator is equal to or larger than the coherent length of the laser, the output beam of the isolator can effectively become unpolarized. Even where the optical path length difference introduced by the optical isolator is smaller than the coherent length of the laser, the isolator will still introduce a phase difference between the extraordinary and ordinary rays where the phase difference can be temperature and wavelength dependent. It is therefore desirable to provide an optical isolator whose two optical paths have the same time delay which therefore preserves the polarization state of the light in the forward direction of the isolator. Where the input light is a mixture of polarized and unpolarized light, the polarization preserving optical isolator would preserve also the degree of polarization of the light.